Such a circuit is known from Swiss patent No. 658 908. This circuit serves e.g. for detecting vibrations of a rotary machine that can be a motor of an airplane or any other vibrating structure. It allows to evaluate the quality of the circuit, that means to find faults as for instance of the circuit, that means to find faults as for instance interruptions, short-circuits, bad insulation, intermittent connections at or near to the sensor as well as in the transmission circuit. Moreover, it is also possible to detect the influences of external interference signals produced by electromagnetic coupling, electrostatic coupling or by a ground loop. In general terms, a transmission line between the transducer and the measuring circuit can be interrupted by one or several connectors at passages through walls located between the piezoelectric transducer and the measuring device for vibrations or pressures.
In spite of the advantageous possibilities that the above mentioned circuit offers, it suffers from an important disadvantage. The test signals and the fault indicating signal respectively are only produced when the device to be monitored, e.g. a motor, is in operation. This is the case, because the vibrations of the device to be monitored produce in the transducer both the vibration measurement signal and the test signal for testing the operation of the measuring circuit. The requirement to operate the device in order to test it is particularly inconvenient e.g. for testing the vibration measuring circuit of a motor of an airplane or a great machine like a gas or vapor turbine which has to be put in operation in order to obtain a test result.
The aim of the present invention is to provide a complete surveillance of the vibration measuring circuit of a machine not only when the machine is in operation, but also when the machine is at rest.
This aim is attained with a measuring circuit of the above mentioned kind, wherein said measuring circuit comprises means for injecting an auxiliary signal to the terminals of the transducer.
By injecting an xe2x80x9cartificialxe2x80x9d auxiliary signal one simulates a known noise in common mode that is used for testing the measuring circuit. Moreover, the following advantages are obtained:
The auxiliary test signal is independent from the amplitude and the frequency of vibration,
the parameters such as frequency and amplitude are foreseeable,
the presence of the transducer can be detected.